1. Field of the Invention
The invention relates to an optical axis inspection method and optical axis inspection apparatus for inspecting whether or not an optical axis of a projection type light source device of a vehicle lighting apparatus or an optical axis of a headlamp provided with the projection type light source device is proper. Especially, the invention relates to an optical axis inspection method and optical axis inspection apparatus for inspecting whether or not an optical axis is proper when a light distribution pattern of a projection type light source unit or a headlamp, which is an object to be inspected, projected onto a screen is captured and subjected to image processing so as to find a cutoff line and the thus found cutoff line is compared with an acceptance reference cutoff line.
2. Background Art
Patent Documents 1 and 2 disclose an optical axis inspection method and optical axis inspection apparatus in which: a light distribution pattern of a headlamp, which is an object to be inspected, projected onto a screen is captured; a cutoff line of the light distribution pattern is found by an image processing unit; both the acceptance reference cutoff line and the cutoff line described above are displayed on a display unit; it is judged whether or not an optical axis is proper by whether or not both the cutoff lines (images) agree with each other; and when the optical axis is not proper, a position of the headlamp is adjusted so that both the cutoff lines (images) can agree with each other.
Patent Document 3 discloses a vehicle headlamp in which: a plurality of projection type light source units (Each light source unit includes a light emitting element, a reflector, a shade for forming a cutoff line and a projection lens.), the light sources of which are light emitting elements, are integrated into a single light source unit assembly arranged in parallel with each other. When light is emitted from this light source unit assembly, a low beam is formed. This light source unit assembly is composed in such a manner that a plurality of light source units are assembled and integrated into a single body so that the respective optical axes can be arranged in parallel with each other on a front face side of a lamp bracket. When light distribution patterns formed by the respective light source units are synthesized with each other, a light distribution pattern of a low beam of the headlamp is formed.    [Patent Document 1] JP-A-59-024232    [Patent Document 2] JP-A-63-113339    [Patent Document 3] JP-A-2005-166588
In the light source unit assembly in Patent Document 3, all optical axes of the projection type light source units composing the light source unit assembly are designed to be in parallel with each other. That is, all optical axes of the projection type light source units are designed so that the cutoff lines of the light distribution patterns of the light source units can agree with each other. However, the following problems may be encountered. For some reasons, by an error caused at the time of assembling, the optical axes of the light source units are not parallel with each other and the cutoff lines of the light distribution patterns of the respective light source units do not agree with each other, that is, an improper synthesized light distribution pattern is formed.
Therefore, before the light source unit assembly is assembled as a headlamp, it is necessary to inspect whether or not all the optical axes of the respective light source units are parallel with each other, that is, it is necessary to inspect whether or not the cutoff lines of the light distribution patterns of the light source units agree with each other. The light source unit assembly is deliberately manufactured so that the accuracy of parts related to the optical axes in the upper, lower, right and left direction of the light source units can be the same with each other. For example, allowable values of the deviation of the optical axes of the light source units are set at ±0.1° in the vertical direction and ±0.13° in the lateral direction. As described above, the allowable values in the vertical direction, which are directly connected to the visibility and the glare light given to an opposed car, are more severely set than the allowable values in the lateral direction. The reason is that when the cutoff lines in the vertical direction of the light distribution patterns of the light source units agree with each other, that is, when the optical axes are parallel with each other in the vertical direction, the optical axes in the lateral direction agree with each other, that is, the optical axes in the lateral direction become parallel with each other.
Therefore, an inventor of the invention considered as follows. Illumination light of each light source unit is introduced onto a screen. A light distribution pattern projected on the screen is taken in as an image by a television camera and subjected to image processing. By the above processing, a cutoff line of the light distribution pattern of each light source is found. A shift in the vertical direction of the cutoff line of the light distribution pattern made by the light source unit with respect to the reference cutoff line is measured. In this case, the reference cutoff line is, for example, a cutoff line of the light distribution pattern made by the light source unit for illuminating mainly a neighborhood of a hot zone. From the thus measured shift, it is possible to judge whether or not the optical axes of the light source units composing the light source unit assembly are proper, that is, it is possible to judge whether or not the optical axes of the light source units are parallel with each other.
Therefore, the inventor started developing an inspection method and an inspection apparatus for inspecting whether or not the optical axes of the light source unit assembly, the stricture of which is shown in Patent Document 3, are proper, that is, whether or not the optical axes of the light source unit assembly, the structure of which is shown in Patent Document 3, are parallel with each other, by utilizing the prior art (the optical axis inspection apparatus) disclosed in Patent Documents 1 and 2. In this developing process, the following problems were encountered. Cutoff lines of the light distribution patterns of the light source units, which were projected on the screen, were not clear. Therefore, it was impossible to accurately find the cutoff line by the image processing. Accordingly, the accuracy of the optical axis inspection could not be enhanced.
That is, the following were found. In a light that is passing thorough the projection lens and directed to the screen, the light passing through an upper portion or a lower portion of the lens, the thickness of which is suddenly changed on a vertical cross section of the lens, is greatly affected by chromatic aberration. Therefore, a red and a blue lamination line (An upper portion is red and a lower portion is blue.) are made to appear along the cutoff one on the light distribution pattern projected on the screen and further, blur is generated on the cutoff line. Therefore, the cutoff line on the light distribution pattern formed by the light through the projection lens becomes unclear. Accordingly, it is impossible to find an accurate cutoff line by the image processing.